Vacuum tube circuits



Feb. 13, 1951 P. N. MARTIN 2,541,879

VACUUM TUBE CIRCUITS Filed July 18, 1944 0900618129 Unz'ef.

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Patented Feb. 13, 1951 vacuum TUBE cmourrs Paul N. Martin, Penn Township, Allegheny County, Pa., islignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application July 18, 1944, Serial No. 545,519

. Claims. 1

My invention relates to vacuum tube circuits, and more particularly to vacuum tube circuits for use withrailway track circuits.

A bias voltage is often provided for a vacuum tube to give a desired operating characteristic for the tube. For example, a bias voltage is applied to a gas tube to preselect the firing voltage of the tube. The use of gas tubes in safety circuits requires that the bias voltage be checked because the loss of a bias voltage might result in a tube being fired when no control is applied thereto. Furthermore, in systems using alternating current for the source of power for gas tubes, proper phase relationship of the currents for the different circuits is essential.

Accordingly, a feature of my invention is the provision of novel and improved vacuum tube circuits.

Another feature of my invention is the provision of vacuum tube circuits incorporating novel means to check the bias voltage applied to a tube.

Another feature of my invention is the provision of track circuit apparatus incorporating novel and improved means to check the bias voltage of a gas tube used as a track relay and to assure proper phase relationship for the voltages of the difierent tube circuits.

Other features. objects and advantages embodying my invention will appear as the speciflcation progresses.

The foregoing features, objects and advantages embodying my invention are accomplished by a circuit arrangement which includes the shield grid or control electrode and the source of bias voltage in series in a bias circuit, and through which circuit power for the anode circuit is obtained. In this way a loss of the bias voltage or a broken element of the bias circuit results in a loss of power for the anode circuit and the tube cannot be fired in response to such failure. The control grid circuit includes means through which the phase relationship of the voltage applied to the control grid can be preselected and thus proper phase relationship between the voltage and the control grid circuit and the voltages of the anode and shield grid circuits can be predetermined.

I shall describe three forms of apparatus embodying my invention, and shall then point out the novel features thereof in claims.

In the accompanying drawings, Fig. 1 is a diagrammatic view showing one form of vacuum tube circuit embodying my invention when used in a railway track circuit supplied with alterhating current. Figs. 2 and 3 are diagrammatic views showing two different forms of vacuum tube circuits embodying my invention when used in ttrack circuits supplied with coded direct curren 1 It is to be understood that my invention is not limited to railway track circuits and this one use illustrates my invention there being many other places the apparatus is useful.

In each of the different views like reference characters are used to designate similar parts.

Referring to Fig. 1, the reference characters Ia and lb designate the track rails of a stretch of railway track formed by the usual insulated rail joints with a track section D-E, and which section may be one section of a series of sections of a signal system. Section D-E is provided with a track circuit comprising a source of current connected across the rails at one end of the section and a track relay connected across the rails at the opposite end of the section. The immediate source of current for the track circuit of section D-E is a track transformer TE, a primary winding 6 of which is connected to an alternating current transmission line including line wires LI and L2 connected to a generator, not shown. A secondary winding I of transformer TE is connected across the rails Ia and ID, a current limiting impedance 8 being preferable interposed in the connection.

The track relay for section D--E comprises a. vacuum tube VI and associated circuits. Tube VI may take different forms, and it is disclosed as an indirectly heated gas tetrode having an anode 9, a cathode III, a control grid II and a shield grid I2. Tube VI is provided with two terminals for the shield grid I2, each end of the grid I2 being connected to a terminal. Thus, a circuit connected to the two terminals of the shield grid includes the shield grid in series so that any break in the shield grid or its connections within the tube will interrupt the circuit. v The filament of the tube VI is heated from a secondary winding l3 of a line transformer TL, a primary winding it of which transformer is connected across the transmission line.

Tube VI is provided with a control grid circuit receiving energy from the track rails through a track transformer TRD and a phase shifting impedance. Specifically, a primary winding 2 of transformer TRD is connected across the rails Ia and lb through wires 61 and 68, a resistor 69 being preferable connected in multiple with winding 2. Secondary winding 4 of transformer TRD is included in the control grid circuit which extends from grid ll through resistor I5 to the junction terminal of an adjustableinductance l6 and a resistor connected in series across the outside terminals of winding 4, and from an intermediate terminal of winding I through a bias battery II and wire II to cathode Ill. Thus the current supplied to the rails through track transformer TE causes a voltage to be applied to the control grid ll of tube VI, the phase of the voltage applied to the grid with respect to the phase of the voltage across the rails being predetermined through the inductance l9 and resistor I. The resistor 99 is provided in order to limit the amount of phase shift with changes in ballast conditions, but such resistor may not be needed. Also, the bias battery II is provided to give a normal negative bias voltage, but such bias battery may not be required.

Shield grid or control electrode I2 is included in a biasing circuit that can be traced from the right-hand terminal of secondary winding ll of line transformer TL through wire it, primary winding l9 of a transformer TP, wire 20, one terminal of the shield grid l2, the shield grid II, the other terminal of the grid and wire 2| to the left-hand terminal of winding I1. is connected to cathode ll of the tube through wires 29 and I9, and thus the shield grid I2 is of a potential with respect to cathode l9 proportional to the voltage of secondary winding I I. An anode circuit for tube VI extends from the top terminal of secondary winding 22 of transformer TP through a battery 12, resistor 23, winding of a relay CR, wire 24, plate 9 and intervening tube space to cathode ill of tube VI and wires 19 and to the other terminal of winding 22. It is to be seen that secondary winding ll of the line transformer TL not only supplies a bias voltage for the shield grid [2 with respect to cathode II but also supplies power through transformer TP to the anode circuit of the tube. Thus a loss of power at secondary winding H or a broken circuit element for the biasing circuit of shield grid l2 will result in a loss of power for the anode circuit of the tube.

Plus and minus signs have been placed on certain of the circuit elements of Fig. 1 to indicate the relative polarity of the parts during one half cycle of the voltage of the transmission line LIL2, the relative polarity of these circuit parts being reversed from that indicated during the other half cycle of the voltage. During the half cycle, the voltage is that indicated by the plus and minus signs, the shield grid I2 is negative in potential with respect to cathode l9 and the anode 9 is positive in potential with respect to the cathode. The parts are so proportioned that the higher voltages of such half cycle applied to the anode is suflicient to fire the tube. except for the negative bias effected through the shield grid [2. It is to be pointed out that battery 12 is of a voltage insumcient to fire tube VI and serves to aid in preselecting the portion of the cycle of the alternating voltage supplied through transformer TP the tube is conductive. The battery 12 may not be needed and may be omitted.

Normally, that is, when the track section D-E is unoccupied, the control grid II is driven positive in potential with respect to cathode l0 during the half cycle the anode 9 is positive and such positive bias voltage for the control grid I l causes tube VI to be fired and relay CR energized by the anode current. During the following half cycle of the voltage the anode 9 is negative with respect to the cathode Ill and the tube is de- Wire l9 ionized. On the next positive half cycle the tube is fired again so that relay CR is energized by impulses of the anode current and its armature is retained picked up as long as such impulses of current flow because of the slow release characteristic of the relay. It is to be observed that the proper phase relationship of the voltage applied to the control grid II is preselected by the adjustment of the inductance l9 and resistor I.

When the'track section D-E is occupied, the control voltage impressed across control grid II and cathode It is shunted and the tube VI remains non-conductive because of the bias voltage applied to the shield grid l2.

It is apparent that in Fig. 1 there is disclosed a track circuit using a gas tube as a track relay and through which tube a control relay CR is retained energized in response to the track section being unoccupied, and is deenergized when the section is occupied. Furthermore, a loss of power at winding ll of the line transformer so that there is a loss of bias voltage for the shield grid l2 results in a loss of power for the anode circuit and the tube cannot be fired because of such failure. Also any broken circuit element for the shield grid circuit results in a loss of power for the anode circuit and the tube cannot be fired because of such a broken circuit element.

In Fig. 2, the rails Ia and lb are formed with a track section D-E the same as in Fig. 1. Section D--E of Fig. 2 is provided with a track circuit, the sourcev of power of which is a battery BE connected across the rails through a contact ltlla of a code transmitter llillCT. Code transmitter IBIICT may be of any one of the several constructions known to the art and it is sufficient for this application to point out that the coder IBOCT is constructed to actuate a contact member "0b to recurrently engage contact Illa at a preselected code rate such as, for example, times per minute, the arrangement being such that contact I990 is closed substantially one half of each code period. It follows that the track circuit current for section D-E of Fig. 2 is a coded direct current of a preselected code rate.

The track relay means responsive to the coded direct current of the track circuit of Fig. 2 includes gas tube Vi and a code following relay CF together with associated circuits. The shield grid l2 of tube VI is included in a biasing circuit extending from the positive terminal of a battery 26, through resistor 21, winding of a check relay CC, resistor 29, one terminal of shield grid l2 to its other terminal and wire 29 to the negative terminal of battery 29. Cathode ll of tube VI is connected through wire 99 to an intermediate terminal 9! of the resistor 28, and it is clear that shield grid I2 is made negative in potential with respect to cathode It by a voltage preselected by the position of the intermediate terminal 3|. Also the check relay CC, which is included in series with the shield grid I2 is ener gized and picked up by the current flowing in this circuit, but that any broken connection or circuit element of the circuit results in the deenergizing of the check relay.

An anode circuit for the tube VI of Fig. 2 can be traced from the positive terminal of battery 26, through resistor 21, front contact 32 of the check relay CC, winding of code following relay CF, wire 33, anode 9 and tube space to cathode ll of the tube, wire 30 to intermediate terminal 3| of resistor 28 and thence through the shield grid circuit to the negative terminal of battery 2..

anaero- Thus the anode I is made positive in potential with respect to the cathode II when the shield grid circuit is complete so that the check relay CC is energized. The parts are so proportioned that the voltage applied to the anode 9 is sumcient to fire the tube except for the negative bias voltage applied to the shield grid I2. Tube VI of Fig. 2 is provided with a condenser CI and a resistor 34 together with a shunt path by which the condenser CI isconnected across the anode 3 and cathode II) through a front contact 35 of the code following relay CF.

The control grid II and cathode III are connected across the rails of the section D-E through a circuit includingresistors 69 and I5.

Specifically, resistor 69 is connected across the rails, grid II is connected to an intermediate terminal of resistor 53 through the resistor I5 and the cathode I0 is connected to an outside terminal of the resistor 69.

Normally, that is, when the track section D-E of Fig. 2 is unoccupied, a control voltage is applied to the control grid II of tube VI each on period of the coded track circuit current due to the voltage drop across resistor 69, the control grid II being driven in the positive direction with respect to the cathode ID. This control voltage applied to the control grid I0 counteracts the negative bias voltage applied to shield grid I2 and the tube VI is fired causing the code following relay CF to be energized and picked u by the current flowing through the anode circuit. Prior to the picking up of the relay CF, the condenser CI is charged through resistor 34 and the tube,

the right-hand terminal of condenser CI as viewed in Fig. 2 being the positive terminal. With relay CF picked up to close front contact 35 condenser CI is connected directly across the anode and cathode of the tube VI to deionize the tube with the result relay CF is deenergized and released. The parts are so proportioned that the on period of the coded track circuit current is slightly less than the time required for the relay CF tobe picked up and for condenser CI to deionize the tube. released and the tube is conditioned ready to be fired on the next on period of the track circuit current and such operation will be repeated as long as the track circuit is unoccupied. When the section D-E is occupied, the control voltage ,applied to the control grid II is shunted and tube VI remains non-conductive because of the negative bias voltage applied to the shield grid I2.

It is to be seen, therefore, that as long as section DE is unoccupied, relay CF is operated at a rate corresponding to the code rate of the track circuit current and with relay CF operated to alternately close front contact 36 and back contact 31 current impulses of a code 'rate corresponding to the code rate of the track circuit current are applied to a decoding unit shown conventionally at I80DU to energize a control relay AA. Decoding unit I8IIDU may be of any one of the several constructions known to the art and is shown conventionally since its construction forms no part of my invention.

It is apparent thatin Fig. 2 a loss of power at battery 28 so that there is a loss of bias voltage for the shield grid results in a loss of power for the anode circuit. Also a broken circuit element of the shield grid circuit results-in deenergizing checkrelay CC to interrupt the anode circuit. Thus in Fig. 2 the tube VI cannot be fired due to a loss of bias voltage.

Referring to Fig. 3, the track rails la and lb Consequently, the relay CF is are formed with a track section DE provided erning a polar stick relay PS, the arrangement being such that when coder contact 38 is closed, a top winding of relay PS is energized to operate the armature of relay PS to the left to close polar contact 40 and when coder contact 33 is closed, a lower winding of relay PS is supplied with current to operate the armature of that relay to the right to close polar contact 4I. Thus when code transmitter I80CT operates to close its front contact 38 there is a brief period before polar relay PS is operat;d to open its right-hand polar contact 4| and when code transmitter I'8IICT is operated to close contact 39 there is a brief period before relay PS is operated to open its lefthand contact 40. During the brief period the coder is in position to close its contact 38 and the right-hand polar contact H of relay PS is closed, current is supplied to the track circuit from battery 42, current flowing through contact 43 of the code transmitter, wire 44, rail la. the track relay or the train shunt if the section is occupied, rail lb, wire 45, and right-hand polar contact H to battery 42. This impulse of current I shall consider as being of positive polarity. During the brief interval the code transmitter is in a position to close contact 39 and the polar contact 40 of relay PS is closed an impulse of current is supplied from battery 41, the circuit extending through contact 46 of the code transmitter, wire 44, rail Ia, track relay or train shunt to rail Ib, wire 45, and polar contact 40 to battery 41. This code impulse of current I shall consider as being of negative polarity. It follows that for each code cycle of the code transmitter two code impulses are supplied to the track circuit, the impulses being of opposite polarity and each of relatively short duration.

The track relay means for Fig. 3 includes two gas tubes V2 and V3 of any suitable type such as an indirectly heated tetrode. Each tube V2 and V3 is provided with two terminals for the shield grid in the manner explained in connection with tube VI of Figs. 1 and 2. The tubes V2 and V3 have their filaments heated in any convenient manner. v

Each tube V2 and V3 is provided with a combined shield grid and anode circuit, such circuit for tube V2 extending from the positive terminal of a battery 48, through resistor 59, anode 60 and tube space tocathode 53 of tube V2, a bias resistor 55, one terminal to the other of shield grid 52 of tube V3, wire 5|, one terminal to the other of shield grid of tube V2 and to battery 48. Similarly, the circuit for tube V3 extends from positive terminal of battery 48 through resistor 55, anode BI and tube space to cathode 54 of tube V3, resistor 55, shield grid 52, wire 5| and shield grid 50 to the negative terminal of battery 48.

It is to be seen that with either tube conductive the current flows through resistor and creates a voltage drop which serves as a negative bias voltage for the shield grids of the tubes. In other words, the anode current of either tube serves to create a negative bias voltage for the other tube. If tube V2 is conductive the voltage drop across resistor 55 due to the anode current of tube V2 renders shield grid 52 of tube V3 negative in potential with respect to cathode 54, and if tube V3 is conductive the voltage drop created across resistor '55 due to the anode current of 7 tube V3 renders shield grid 80 of tube V2 negative in potential with respect to cathode II. This voltage drop across resistor ll is also applied to the shield grid of the conductive tube but is ineilective to control that tube due to the characteristics 01' gas tubes. Ii tubes v2 and VI are both conductive the voltage drop across resistor I is correspondingly increased but is inefi'ective under such conditions. The parts are so proportioned that battery is is of a voltage suificient to fire each tube were it not for the bias voltage applied to the shi.ld grid through resistor 56. It is to be noted that with neither tube conductive there is no bias voltage applied to the shield grids. In practicing the invention the tubes v2 and VI would be arranged for one or the other or the tubes to be initially conductive so.

that the shield grid of the other tube would be biased to maintain such other tube non-conductive until a control voltage is applied to such other tube in a manner to be explained shortly. As an alternative arrangement resistor 85 can be replaced by a battery.

A condenser CI is connected across the anodes oi the tubes V2 and V3 and serves to alternately deionize the tubes in the well-known manner when the tubes are alternately fired through control voltages alternately applied to the control grids of the tubes.

Resistor 69 is connected across the rails the same as in Fig. 2, and a right-hand portion of resistor 89 is included in a control grid circuit for tube V2 and a lei't-hand portion of resistor is is included in a control grid circuit for tube VI, as will be readily understood by an inspection of Fig. 3. Thus, when the section D-E is unoccupied each code impulse of the track circuit current creates a corresponding voltage drop across resistor 69, the polarity being according to the polarity oi the track circuit impulse.

In describing the operation of the apparatus of Fig. 3, I shall assume that at the start the tube V! is conductive and tube Vlis non-conductive, section D-E is unoccupied and the first code impulse of the track circuit current isone that causes the right-hand terminal of resistor 69 to be positive. Under such circumstances the condenser CI is charged by the voltage drop across resistor 58, the left-hand terminal of the condenser being the positive terminal. Then in response to the code pulse of track circuit current the control grid 62 of tube V2 is driven in the positive direction by the voltage drop across the right-hand portion of resistor 89. This control voltage opposes the bias voltage of shield grid 80 of tube V! and the parts are so proportioned that such control voltage permits tube V! to be fired. With both tubes V2 and v: conductive. the charge oi condenser CI is applied across the tubes in series and the polarity of the charge is such that tube V! is deionized. Subsequent to the deionizing of tube V3 the condenser CI discharges through the circuit extending from the positive left-hand terminal of condenser Cl through the anode to cathode space of tube V2, resistor 65, the shield grids it and ill and connecting wire 5i, battery l3 and resistor 56 to the negative right-hand terminal of condenser C3. Subsequent to the discharge of condenser Cl it is recharged due to the voltage drop across resistor is caused by the conductive condition of tube V2, the right-hand terminal of condenser Cl being the positive terminal. The next code impulse oi the track circuit cur.-

rent is 01' reverse polarity, and the voltage drop across resistor 88 causes its left-hand terminal to be positive and a control voltage is applied to control grid 84 of tube V3 to drive grid 84 in the positive direction in opposition to the bias of the shield grid 52. Such control voltage applied to tube VI causes that tube to be fired. with both tubes conducting, the charge or condenser C3 is applied across the tubes and the polarity of the charge is such that tube V! is deionized. Subsequent to the deionizing of tube V2, the condenser Cl discharges through the circuit extending from the positive right-hand terminal 01 condenser CI through anode to cathode tube space of tube V8, resistor I6, shield grids 52 and 50, battery 48 and resistor I! to the negative lefthand terminal of condenser Cl. Condenser C3 is now recharged with its left-hand terminal the positive terminal due to the voltage drop across resistor It. The next impulse of track circuit current is one that causes the right-hand terminal oi resistor 69 to be positive and tube V! is fired in the manner explained hereinbeiore. The charge of condenser CI now is applied to the tubes to deionize tube V3, and subsequent to Y the deionizing oi tube V! the'condenser Cl is first discharged and then recharged with its right-hand terminal positive in the manner explained hereinbeiore.

It is to be seen from the foregoing that the impulses oi the track circuit current alternately fire the tubes V2 and V3 and an eiectromotive iorce alternating in polarity at a rate corresponding to the code rate of the impulses of the track circuit current appears across the outside terminals oi. resistors 58 and I9. This electromotive force appearing across resistors 56 and 59 is applied to a decoding unit IOODU tuned to effectively energize a control relay AA when the polarity of the eiectromotive force alternates at a frequency corresponding to the code rate of the track circuit current.

when the section D-E is occupied, the control voltage created across resistor 69 is shunted and the alternate operation of the tubes V2 and VI ceases with one or the other of the tubes left conductive.

It is apparent that in Fig. 3 a loss of power that will cause a loss of the bias'voltage applied to the shield grids 50 and 52 will also cause a loss of power to the anode circuit. Also, any broken connection in the combined shield grid and anode circuit of either tube V! or V3 will open such anode circuit and the tubes cannot be fired.

It is to be understood that my invention is not limited to a shield grid as the control electrode oi a tube to be provided with two terminals and included in series with a bias voltage source and other tube electrodes can be used.

Although I have herein shown and described but three forms of vacuum tube circuits embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention. what I claim is:

1. In combination; a tube having a sealed envelope containing an anode, a control electrode and a cathode; an anode circuit connected across said anode and cathode and including a source of current effective to fire the tube, a control circuit including a source oi bias voltage and 9 connected to said control electrode and cathode with the source of bias voltage poled to bias the control electrode negative in potential with respect to the cathode to render said anode circuit ineffective to fire the tube, and said tube having two control electrode terminals one connected to each end oi the electrode and said control circuit connected to said terminals to include the control electrode in series with said source of bias voltage to check the continuity of the control electrode.

2. In combination; a gas type tube having an anode, a cathode and a shield grid; said shield grid provided with two terminals one connected to each end of the grid, an anode circuit connected across said anode and cathode and including a source of power effective to fire the tube, a shield grid circuit including a bias voltage source connected to said shield grid and cathode ,with said bias voltage source poled to bias the shield grid negative in potential with respect to said cathode to render the anode circuit ineffective to fire the tube, said shield grid circuit connected to both of said two terminals to include the shield grid in series in the circuit for current to normally flow in the shield grid circuit, and said shield grid circuit electrically coupled to said anode circuit in such a manner that said power source is active to energize the anode circuit only when the shield grid circuit is closed whereby firing of the tube due to a failure of the shield grid circuit is avoided.

3. In combination; a gas type tube having an anode, a cathode, a control grid and a shield grid; a source of alternating current, a transformer having a first and a second winding; a biasing circuit including in series one terminal of said alternating current source, said first transformer Winding, a connection to said shield grid, and the other terminal of the alternating current source; said cathode connected to said biasing circuit intermediate said alternating current source and said first transformer winding to create a given bias voltage of said shield grid with respect to said cathode due to the current of the biasing circuit flowing in said first transformer winding, an anode circuit including said second transformer winding for receiving power from said biasing circuit and connected across said anode and cathode to fire said tube but normally ineffective due. to said bias voltage, a control grid circuit including a source of control voltage and connected to said control grid and cathode to at times create a voltage of the control grid with respect to the cathode to counteract said bias voltage at the shield grid to enable said anode to fire the tube, and control means interposed in said anode circuit energized by the current thus caused to flow in the anode circuit.

4. In combination, a gas type tube having an anode, a cathode, a control grid and a shield grid; 9. source of alternating current, a first secondary windings connected for said shield grid to be negative in potential with respect to said cathode during the half cycle of the alternating current said anode is positive in,- potential with respect to the cathode to normally avoid firing of the tube, and control means connected across said control grid and cathode to render at times said control grid positive in potential with respect to said cathode to counteract said shield grid bias to fire the tube each half cycle the anode is positive.

5. In combination; a gas type tube having an anode, a control electrode and a cathode; an anode circuit connected across said anode and cathode and including a current source effective to fire the tube, a check relay, a biasing resistor; a biasing circuit including in series said current source, a winding of said relay, said biasing resistor and said control electrode; said check relay being energized by the current thus flowing in such biasing circuit, said cathode connected to said biasing resistor in such a manner as to bias the cathode positive in potential with respect to said control electrode to render the anode circuit ineffective to fire the tube, and a front contactof said relay interposed in said anode circuit to avoid firing of the tube due to a failure of said biasing circuit.

6. In combination; a gas type tube having an anode, a cathode, a control grid and a shield grid; a check relay, a source of current; a shield grid circuit including in series said current source, a winding of said relay, and said shield grid to energize said relay; said cathode connected to said shield grid circuit for said shield grid to have a preselected negative bias voltage with respect to said cathode, an anode circuit connected to said anode and cathode and including a front contact of said check relay and a winding of another relay to fire the tube but normally inefiective due to said bias voltage, control means connected to said control grid and cathode and operable to supply code impulses of current to counteract-said bias voltage each such code impulse to fire the tube and energize said other relay, capacitance means normally charged through said anode circuit and connected across said anode and cathode to deionize said tube whereby said other relay is operated at a code corresponding to said code impulses, and. means including contacts of said other relay energized in response to such operation of said other relay.

7. In combination; a gas type tube having an anode, a cathode, a control grid and a shield grid; said shield grid provided with two terminals onelconnected to each end of the grid, a battery, a source of bias voltage; a combined anode and shield grid circuit including in series the positive terminal of said battery, anode to cathode tube space of said tube, said source of bias voltage, one terminal of said shield grid the shield grid, the other terminal of the shield grid and the negative terminal of said battery; said source of bias voltage disposed to provide a preselected negative bias' voltage of the shield grid with respect to said cathode, said battery disposed to render said anode positive in respect to the cathode to fire the tube but normally ineffective due to said bias voltage of the shield grid, and control means connected across said control grid and cathode to render at times said control grid positive with respect to the cathode to counteract said shield. grid bias voltage to permit said anode circuit to fire the tube, and means 11 coupledtosaidanodecircuitbythe current thus caused to fiow in the anode circuit.

8. In combination; a first and a second gas tubeeachhavingananode,acathodeanda control electrode; a source oi direct current. a circuitforeachofsaidtubesandeachofwhich circuits includes said current source and the anode to cathode tube space of the respective tube, said current source being of a voltage sufficient to fire each tube. and a resistor made commontosaidcircuitsandconnectedtosaid control electrodes for the current fiowing in either tube when conductive to bias the control electrode of the other tube negative in potential with respect to the cathode of said other tube to render said current source ineffective to fire such other tube.

9.Incombination;afirstandasecondgas tube each having an anode, a cathode and a control electrode; a source of direct current. a bias resistor; a combined control electrode and anode circuit for each tube: each of said circuits including in series said current source, the

anode to cathode tube space of the respective tube, said bias resistor and the control electrodes of the two tubes; and said bias resistor disposed in said circuits for the voltage drop created across the resistor by the current flowing in either of said combined control electrode-anode circuits due to the firing of a tube to bias said control electrodes negative in potential with respect to said cathodes.

10. In combination, a tube having a sealed envelope containing an anode, a cathode and at least one control electrode, said tube provided with two terminals for said control electrode one connected to each end of the electrode, a source of power electromotive force, a source of bias electromotive force: a tube circuit including in series said source of power electromotive force, said anode and tube space to said cathode, said source of bias electromotive force, a first one of said terminals, said control electrode and a second one of said terminals, and said bias source of electromotive force and said power source of electromotive force disposed to create a preselected conductive condition for said tube and which condition is checked by said series arrangement of the tube elements in said tube circuit.

11. In combination; a gas tube having an anode, a cathode and at least one control eiectrode; said tube having two terminals which are connected one to each end of said control electrode to include the control electrode in series between the two terminals, a source of power voltage, a source of bias voltage; a tube circuit including in series said source of power voltage, said anode and tube space to said cathode, said source of bias voltage, one of said terminals, said control electrode and the other one of said terminals; and said source of bias voltage disposed to bias said control electrode negative in potential with respect to said cathode to render said source of power voltage ineffective to fire said tube and which non-conductive condition is checked by the series arrangement of sai tube circuit. a

12. In combination; a gas tube having an anode, a cathode and a first and a second grid; said tube provided with two terminals for said first grid one connected to each end of that grid, a source of power voltage, a source of bias voltage, a load element; a tube circuit including in 1 series one terminal of said source of power volttive force, said circuit network provided with connectionstosaidanode,cathodeandtosaid twoterminalsofsaldcontrolclectrodetorender the anode positive in potential with respect tcsaidcathodeandtobiassaidcontrolelectrodc negative in potential with respect to the cathode, and said bias potential preselected to maintain a given conductive condition of the tube for said anode potential to effect a given value or anode current and which conductive condition of the tube is checked by said series arrangement of the control electrode between its two terminals.

14. In combination; a gas tube having an anode, a cathode and a shield grid; transformer meanshavingailrst,asecondandathird winding; said first winding energised from an alternating current source; a biasing circuit including one terminal of said first winding, said secondwindinmaterminalofsaidshieldgrid andtheotherterminalofsaidfirstwinding; saidbiasingcircuitbeingacicsedcircuitin which current normally fiows due to the energizingofsaidfirstwindinmsaidcathodeconnectedtosaidbiasingcircuitintermediatesaid firstandsecondwindingsforthecathodeto have a given voltage with respect to said shield gridduetothevoltagedropinsaidsecond wlnding.ananodecircuitincludingsaidthird windingandconnectedacrosssaidanodeand' cathode,saidthirdwindingcoupledtosaidseeand winding to receive a voltage due to the biasingcircuitcurrcntfiowinginsaidsecond windimandsaldvoltageofthethirdwindingof said anode circuit eifective to fire the tube but maintained ineffective to fire the tube due to said given voltage of said cathode with respect to said shield grid and which ineffectiveness of the anode circuit is retained when any one of said windings is short-circuited.

15.- In combination; a gas tube having an anode, a cathode, a control grid and a shield grid; said shield grid having two terminals one connectedtocachendofthegrid.analternating current source, an energy transfer means; a biasing circuit including in series said current source, said energy transfer means and said shield grid and its two terminals to effect a current flow through said energy transfer means only when said shield grid is intact; said cathode connected to said biasing circuit intermediatethecurrentsourceandsaidenergytransfer means'whereby the cathode has a bias voltage with respect to said shield grid due to the voltage drop across said energy transfer means, an anode circuit receiving power from said energy transfer means and connected to said anode and cathode for firing the tube but maintained ineflective due to said bias voltage between the cathode and shield grid, and a control grid circuit including a control voltage source and connected across said control grid and cathode for at times applying to the control grid a voltage that counteracts said bias voltage between the shield grid and cathode to permit said anode circuit to tire the tube.

PAUL N. MARTIN.

REFERENCES CITED The following references are of record in the me of this patent:

Number 14 UNITED STATES PATENTS Name Date Wright Oct. 23, 1917 Meyer July 13, 1926 Demarest June 26, 1934 Hertwig July 28, 1936 Buchting Oct. 6, 1936 Cockrell Mar. 29. 1938 Nelson Sept. 3, 1940 O'Hagan Oct. 1, 1940 Sorensen Oct. 1, 1940 O'Hagan Apr. 8, 1941 Hallden Oct. 20, 1942' Carleton Oct. 10, 1944 

